[PATCH 6.1 603/609] sctp: fix use-after-free of cached ASCONF chunk

6.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yuxiang Yang <[email protected]>

commit 8c283e7b56adce00193837f3311b06662466fb21 upstream.

addip_last_asconf caches the outstanding outbound ASCONF chunk. The normal
ASCONF-ACK completion path releases the chunk and clears the pointer.

However, sctp_asconf_queue_teardown() releases the cached chunk without
clearing addip_last_asconf. During peer restart handling,
sctp_sf_do_dupcook_a() queues SCTP_CMD_PURGE_ASCONF_QUEUE, which invokes
sctp_asconf_queue_teardown() while the association remains alive and leaves
the pointer dangling.

A delayed authenticated ASCONF-ACK can then reach sctp_sf_do_asconf_ack(),
which accesses the stale chunk and passes it to sctp_process_asconf_ack(),
causing a use-after-free and a second release.

Clearing the pointer exposes a race with T4 expiry. Peer restart handling
queues the timer stop before the purge, but SCTP_CMD_TIMER_STOP uses
timer_delete(), which does not wait for a callback already running on
another CPU. Such a callback can reach sctp_sf_t4_timer_expire() after
the purge and dereference NULL.

Clear addip_last_asconf after releasing the cached chunk, and make
sctp_sf_t4_timer_expire() consume a stale T4 expiry if no outstanding
ASCONF remains.

Fixes: a000c01e60e4 ("sctp: stop pending timers and purge queues when peer restart asoc")
Cc: [email protected]
Suggested-by: Xin Long <[email protected]>
Signed-off-by: Yuxiang Yang <[email protected]>
Acked-by: Xin Long <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 net/sctp/associola.c    |    4 +++-
 net/sctp/sm_statefuns.c |    6 +++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

--- a/net/sctp/associola.c
+++ b/net/sctp/associola.c
@@ -1732,6 +1732,8 @@ void sctp_asconf_queue_teardown(struct s
 	sctp_assoc_free_asconf_queue(asoc);
 
 	/* Free any cached ASCONF chunk. */
-	if (asoc->addip_last_asconf)
+	if (asoc->addip_last_asconf) {
 		sctp_chunk_free(asoc->addip_last_asconf);
+		asoc->addip_last_asconf = NULL;
+	}
 }
--- a/net/sctp/sm_statefuns.c
+++ b/net/sctp/sm_statefuns.c
@@ -6111,8 +6111,12 @@ enum sctp_disposition sctp_sf_t4_timer_e
 					struct sctp_cmd_seq *commands)
 {
 	struct sctp_chunk *chunk = asoc->addip_last_asconf;
-	struct sctp_transport *transport = chunk->transport;
+	struct sctp_transport *transport;
 
+	if (!chunk)
+		return SCTP_DISPOSITION_CONSUME;
+
+	transport = chunk->transport;
 	SCTP_INC_STATS(net, SCTP_MIB_T4_RTO_EXPIREDS);
 
 	/* ADDIP 4.1 B1) Increment the error counters and perform path failure